Group chats, nicely working and with fallback for legacy clients. They are usable (without all features, of course) in Gajim/etc without doing anything:
Device management. Newly connected device is given a token, with a mechanism to prevent token duplication, and tokens can be revoked, resulting in device disconnection:
- message formatting is a mess
- device management is absent
- iOS apps can't really work with stock xeps
And the list is long, these are just the top problems. Luckily, this will likely change soon.